A CRT (cathode ray tube) video display has the capacity to overdrive its output intensity over small scene areas that are very bright with respect to the rest of the scene. This trait produces an effect that emphasizes locally bright areas with an intensity higher than that achievable over the entire scene. This effect is desirable in many circumstances.
A new projection display that utilizes reflections from hundreds of thousands of micromirrors, each mounted above its own semiconductor memory cell is described in IEEE Spectrum, Nov. 1993, vol. 30, no. 11, written by Jack M. Younse of Texas Instruments Incorporated. The digital micromirror device (DMD) comprises a special light modulator. The DMD, or digital micromirror, device covers each memory cell of a CMOS STATIC RAM with a movable micromirror. Electrostatic forces based on the data in this cell tilt the mirror plus and minus a few degrees, modulating the light incident on the surface. In one example, the tilt is 10.degree.. The light reflected from any of the mirrors passes through a projection lens and creates an image on a large screen. Light from the remaining off mirrors is reflected away from the projection lens and trapped. The portion of time during which each video frame that the mirror remains in the on state determines the shades of grey from black for 0 on time to white for nearly one hundred percent of the time. One of the ways in which color is added is by a color wheel. This type of system is referred to as a sequential DMD video system. A sequential color DMD display that employs a color wheel uses pulse width modulation (PWM) to depict the intensities for each pixel in each wheel color. Individual mirrors are switched between the off and on state so that each is turned on for a portion of the time where the DMD is uniformly illuminated with a single color. The on time for a minor is proportional to desired intensity in that color. Since all available time in one color is used to depict the full scale intensity, a DMD display cannot "overdrive" pixels in particular colors without reducing the amount of time useable for proportional pulse width modulation.